This reminds me of Salvatore's argument against the New Wierd set: that mainstream sales of epic fantasy are what makes the publication of 'fringe work' possible. There has to be some truth to this.
Imagine for a moment that books were no different than any other retail commodity. If that were the case, then we would likely include things like ease of use, stylishness, and reliability among a book's primary virtues. Stylish, easy-to-use, reliable commodities tend to sell.
The thing is many think that books should do more than this, that books should make trends rather than follow them, challenge readers rather than comfort them, and surprise them rather than deliver what they expect - in short, that books shouldn't be stylish, easy-to-use, or reliable.
In other words, that they be entirely unlike most other commodities.
I think it's simply a fact of human nature that the majority prefer things to possess 'commodity virtues' rather than 'book virtues' (though education has the potential, at least, to play a decisive role). I also think that in a consumer society, which is to say a society organized around the production and consumption of commodities, there is always going to be a 'commodity bias' to contend with: this is actually a powerful argument for the necessity of things like public television and public support of the arts.
Because of this, Aldarion, I think the problem you refer too is both inevitable and a matter of degree. (This is also why I think (and I'm referring to past debates here) railing against commercial epic fantasy because they embody commodity rather than book virtues is a non-starter. It's like blaming your mechanic for a mistake made by a design engineer at GM). What makes you think it's getting worse rather than (as I tend to believe) better?